As one exemplary conventional in-vehicle device control system, a vehicular electronic key system (verification system) is disclosed in the following patent document 1. In this vehicular electronic key system, a control apparatus mounted in a vehicle performs checkup about a portable device carried by a user of the control apparatus by mutual radio communication, and controls the in-vehicle device based on a result of the checkup. This vehicular electronic key system includes a smart security control apparatus mounted in a vehicle and a portable device carried by a user of the control apparatus. The smart security control apparatus transmits to the portable device a designation request signal, which designates a response frequency. When no radio signal is received within a predetermined allowable time period after transmission of the designation request signal, the smart security control apparatus transmits a designation request signal by changing the response frequency to a different frequency. The portable device selects the response frequency designated by the designation request signal and transmits to the smart security control apparatus a reception response signal of the selected response frequency. It is prevented that the response signal is not received because of jamming of the response signal. By thus making the radio communication to be less susceptible to influence of factors such as jamming, which disturb radio communication, a control object can be operated normally.
However, in case of transmitting from the vehicle to the portable device at a fixed time interval the request signal, which requests transmission of the response signal, the request signal need be transmitted a plurality of times at the fixed time interval for designating a plurality of response frequencies. To make the radio communication to be less susceptible to influence of factors such as jamming, which disturb radio communication, the number and time of communications will possibly increase, resulting in increased current consumption. The vehicle is at rest in many instances, when the request signal is transmitted from the vehicle to the portable device periodically. If the current consumption increases under this situation, more and more battery power is consumed.